Electrochemical Water Oxidation Catalysis Beginning with Co(II) Polyoxometalates: The Case of the Precatalyst Co4V2W18O6810–sup>
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- 作者：Scott J. Folkman ; Richard G. Finke
- 刊名：ACS Catalysis
- 出版年：2017
- 出版时间：January 6, 2017
- 年：2017
- 卷：7
- 期：1
- 页码：7-16
- 全文大小：482K
- ISSN：2155-5435
文摘
The question is addressed of whether the cobalt polyoxometalate (Co-POM) precatalyst Co<sub>4sub>V<sub>2sub>W<sub>18sub>O<sub>68sub><sup>10–sup> (hereafter Co<sub>4sub>V<sub>2sub>W<sub>18sub>) is a stable, homogeneous water oxidation catalyst under electrochemically driven conditions and in 0.1 M pH 5.8 and 8.0 NaPi buffer as well as pH 9.0 sodium borate (NaB) buffer. This question is of considerable interest since Co<sub>4sub>V<sub>2sub>W<sub>18sub> has been reported to be highly stable and a 200-fold faster water oxidation catalyst than its P congener Co<sub>4sub>P<sub>2sub>W<sub>18sub>O<sub>68sub><sup>10–sup> (hereafter Co<sub>4sub>V<sub>2sub>W<sub>18sub>), for reasons that were not specified. The nature of the true water oxidation catalyst with Co<sub>4sub>V<sub>2sub>W<sub>18sub> as the starting material is of further fundamental interest because a recent report reveals that the <sup>51sup>V NMR peak at ca. −507 ppm assigned by others to Co<sub>4sub>V<sub>2sub>W<sub>18sub> and used to argue for its solution stability is, instead, correctly assigned to the highly stable cis-V<sub>2sub>W<sub>4sub>O<sub>19sub><sup>4–sup>, in turn raising the question of the true stability of Co<sub>4sub>V<sub>2sub>W<sub>18sub> under water oxidation catalysis conditions. A battery of physical methods is used to address the questions of the stability and true water oxidation catalyst with Co<sub>4sub>V<sub>2sub>W<sub>18sub> as the precatalyst: <sup>31sup>P line-broadening detection of Co(II) present in solution from leaching or as a counterion impurity; a check of those Co(II) concentration results by the second method of cathodic stripping; the O<sub>2sub> yield (and, hence, Faradaic efficiency) of electrocatalytic water oxidation; electrochemical, SEM, EDX, and XPS characterization of CoO<sub>xsub> films produced on the electrode; and multiple controls and other experiments designed to test alternative hypotheses that might explain the observed results. The collective evidence provides a compelling case that Co(II) derived from Co<sub>4sub>V<sub>2sub>W<sub>18sub> forms a CoO<sub>xsub> film on the electrode which, in turn, carries all the observed, electrochemically driven water-oxidation catalysis current within experimental error. A list of seven main findings is provided as a summary.